The present invention relates to the recovery of ore from subsurface orebodies by solution mining and, more particularly, to insitu leaching operations wherein the orebody is subjected to the action of a solubilizing agent in order to permit economic extraction of the ore.
At present, solution mining of subsurface orebodies is primarily being utilized to extract uranium values from small orebodies. The attractiveness of the insitu operations is due to a number of factors including lower capital investment, elimination of hazards peculiar to underground mining operations and the substantial reduction in surface and subsidence reclamation costs. As a result, insitu mining by the injection of solutions has permitted the economic exploitation of smaller orebodies.
While all subsurface orebodies are not well-suited for solution mining techniques, successful extraction of ore values has taken place where the subsurface orebody is characterized by an orebody contained within a porous host formation saturated with water and vertically bounded by relatively impermeable strata to provide a confined underground environment. In addition, the host formation should be sufficiently permeable to permit the solutions injected into the formation to flow therethrough Although a number of different techniques and well patterns are used, solution mining utilizes one or more injection wells and one or more recovery wells spaced therefrom with a hydrostatic gradient created therebetween.
Further, the chemical nature of the orebody and the host formation have to be evaluated to determine the type of solubilizing agent and leaching agent needed in the lixiviant to permit the recovery of sufficient ore values to make the endeavor economically feasible. As the lixiviant travels through the host formation, the solubilizing agent, typically an oxidizer, converts the tetravalent uranium to hexavalent uranium. The leaching agent dissolves the hexavalent uranium allowing it to be mobilized and the pregnant solution is then removed from the recovery well for further processing.
Prior activity has been directed to the evaluation of different leaching agents including sulfuric acid, sodium carbonate-bicarbonate solutions and ammonium carbonate-bicarbonate solutions. In addition, hydrogen peroxide, oxygen and different hypochlorates and sulfates have been utilized as the oxidizing agents in a number of applications. The selection of agents typically is made after extensive testing of the orebody and host formation. The agent selection is very important since the ore values are quite low and in order to achieve an economically successful mining operation it is necessary that as much of the ore as practical be exposed to the action of the lixiviant.
In the exposure of the bulk of the orebody to the lixiviant, substantial quantities of contaminants, both minerals and salts, are also contacted by and may be dissolved in the lixiviant. While these contaminants can be separated from the recovered pregnant solution by surface recovery installations, significant quantities of soluble contaminants have remained within the host formation. Typically, the host formation is an aquifer and the ground water contained therein is more than likely found to be contaminated to the point wherein higher than normal concentrations of some ions, particularly uranium ions, exist upon the cessation of solution mining.
It has been shown that a reduction in the level of contaminants in the ground water can be obtained by continued pumping of fluids from the host formation after cessation of the injection of the lixiviant provided adequate water recharge is available. However, this technique alone normally does not restore the ground water to an environmentally acceptable level due to the continued presence of undissolved soluble contaminants throughout surrounding regions of the host formation. These soluble contaminants, notably the uranium, continue to dissolve over a long period of time thereby maintaining the contaminant level of the water at an undesirably high level. The practice of continued pumping also is undesirable in that very large volumes of water may be required for restoration in critical water-limited regions of the United States.
To alleviate these problems it has been proposed that the pumped fluids be restored on the surface and then reinjected. The cycle is repeated with recirculation of a large number of complete pore volumes of the affected portion of the host formation for long periods. As an alternative, U.S. Pat. No. 4,134,618 describes the beneficial effects of cycling clean water through the host formation in order to restore the groundwater to acceptable levels.
The above-mentioned restoration techniques utilize substantial periods of pumping activity interleaved with long shut-in periods. This directly increases the cost of extraction by increasing the time and effort required for post-treatment. Also, the number of pore volumes of fluid required to be recirculated during post-extraction treatment has a direct bearing on the economics of the mining operation. In order to mine small subsurface orebodies, it is clearly desired to both decrease the volume of fluid that must be recirculated and to reduce the duration of the active and shut-in portions of the restoration treatment.